Traffic on optical transport networks requires protection. One way to provide protection is to switch the traffic from a “working” path to a “protection” path if the working path is non-functional for any reason. The switching action will generally result in a brief service disruption for the end customer. To implement this type of protection in a service provider's network, it is important to characterize this protection functionality. Many optical transport network standards (such as GR-253 for SONET) have strict requirements on the maximum service disruption interval, and individual service providers and customers may have even more stringent requirements. This makes it imperative that accurate measurements of the service disruption intervals can be obtained.
Some test sets for measuring service disruption intervals use bit errors in monitored test traffic as the basis for disruption interval measurements. In this type of test set, the test set interprets the first detected bit error as an indication that a service disruption has occurred, and starts a timer. This type of test set assumes that the network protection switching has occurred in response to the first bit error. When the test set has received no other bit errors for a given time period, the test set concludes that the service disruption is now over and stops the timer. The test set reports the elapsed time between the first bit error and when the timer is stopped as the service disruption interval. This method for measuring the service disruption interval requires an error-free idle condition before the protection switch action. If, however, the idle condition is not error-free, this can produce inaccurate or invalid service disruption interval results.